1. Field of the Invention
This invention relates to a platen roller and more particularly to a platen roller adapted for a printer of a typewriter or the like.
2. Description of the Related Art
The printers of typewriters or the like are provided with platen rollers which serve as substrate plates for printing. Generally, the platen roller is manufactured in a cylindrical shape with a rubber member 2 arranged to encompass a core member 1 as shown in FIG. 2 of the accompanying drawings. In printing, for example, a printing paper 3 is brought into tight contact with the circumferential surface of the rubber member 2, as shown in FIG. 3. Then, a print element which is typically represented by a daisy wheel 5 is pushed against the rubber member 2 by hitting the wheel 5 with a print hammer 6 at a part where an ink ribbon 4 is superimposed on the printing paper 3, so that a desired character of the print element can be printed on the printing paper 3.
The printing performance on the printing paper depends on the hammering energy of the printer and the hardness of the rubber member which forms the platen roller. With the hammer energy assumed to be unvarying, the printing performance becomes better accordingly as the hardness of the rubber member increases. Therefore, if the hardness of the rubber member is inadequate for the hammer energy of the used printer, the quality of the print would degrade due to such a defect that a part of the printed character is missing or the contour of the character is obscure.
To avoid such inadequate printing, efforts have heretofore been exerted to find a way as to how to increase the hardness of the rubber member for the platen roller. The methods generally employed for attaining a higher degree of hardness have been, for example, as follows: In one method, the rubber composition is arranged to include at least 20 phr of an organic reinforcer such as a modified melamine resin, a high styrene resin or a phenol resin, for 100 of a polymer. In another method, fillers which are to be contained in the rubber (normally carbon black, an inorganic filler such as a white filler, etc.) are arranged to include, at least 50% by weight, the carbon black of a fine grain size which is at the level of FEF carbon or finer than that.
A vulcanized rubber material which is thus obtained has a desired degree of hardness. However, the rubber material thus obtained generally has an excessively high rebound resilience. Therefore, a platen roller made of such a rubber material causes a great hammer rebound. In other words, only small portion of the whole hammer energy of the printer is consumed for actual printing. In short, it degrades the hammer energy efficiency. It has been thus necessary to make up for the inefficiency by increasing the hammer energy for adequate printing. This causes an increase in the impact load. The wear of the print element thus has been accelerated to result in worn-out types.
To cope with the above stated problem, efforts have been exerted to increase the durability of the print element. For example, in cases where a daisy wheel is employed as the print element, the daisy wheel has been prepared by a so-called dichroic molding process using an expensive phenol resin material for the type part which is apt to wear away and an abrasion resisting material such as polyamide for a part carrying the daisy wheel. However, the problem has not been completely solved as there is a limit to the improvement in the durability. Besides, the conventional solution has presented another problem in terms of economy.